1. Field of the Invention
The present invention relates to a library apparatus having a rotary cell drum with a plurality of cells for accommodating recording medium cartridges.
2. Description of the Related Art
Magnetic tape units are one of diverse kinds of external storage devices for use with computers. The magnetic tape units operate most commonly on what is known as the open reel system using 0.5-inch wide tapes. The chores of manually loading the open reels have been alleviated by widening acceptance of automatic tape loading devices. With these devices, the operator need only set a desired tape reel onto a receptor which then causes the tape to be loaded automatically. Today, operators' burdens are further alleviated by the widespread use of magnetic tape cartridges. A magnetic tape cartridge, accommodating a tape having the same width as that of open reel tapes, allows the tape to be threaded out of the reel within for automatic tape loading.
A plurality of magnetic tape cartridges needed are entered in the magnetic tape library apparatus. Any one of the cartridges is selected and loaded to a magnetic tape drive unit for data recording and reproduction. The magnetic tape library apparatus comprises a cell unit having a plurality of cells each accommodating a magnetic tape cartridge; a magnetic tape drive unit for recording and reproducing data; and an accessor for automatically switching magnetic tape cartridges between cell unit and magnetic tape drive unit. The magnetic tape library apparatus further comprises a cartridge entry unit for entering magnetic cartridges into the library apparatus, and an automatic cartridge exit unit for automatically ejecting magnetic tape cartridges from inside the library apparatus.
The accessor transports magnetic tape cartridges through the magnetic tape drive unit, cell unit, cartridge entry unit and automatic cartridge exit unit. Magnetic tape library apparatuses of this kind are widely used as an external mass storage device for computers. These apparatuses are required to operate unfailingly when unattended.
At present, there exist rotary cell drums as a variation of cell units, the drum being rotated by suitable means. When an accessor is used to store or extract a magnetic tape cartridge to or from a given cell in the cell drum, the cell drum is rotated to and stopped at the appropriate position where the accessor gains access to the target cell. To position the cell drum this way requires rotating it as fast as possible and stopping it at the target position without vibration. The requirements are met conventionally as follows. A flag attached to the cell drum is first detected by a sensor. A detection signal from the sensor then decelerates a cell drum driving motor so that the cell drum will be stopped at the position required.
This conventional method of controlling the cell drum in terms of rotation and stopping has one disadvantage. That is, a reduction gear of a cell drum driving system prevents the cell drum from rotating while a cartridge is being stored or extracted to or from a cell of the drum. An inevitable backlash of the reduction gear would let the cell drum dislodge from its stopped position if left unchecked. In order to keep the cell drum stationary in the target position, it is necessary to supply the cell drum driving motor with a positioning current varying with the detected drum displacement in a compensating manner. The positioning current allows the cell drum to be fine-adjusted in its stopped position. The process of fine adjustment involves an appreciable loss of access time.